I know that the Briggs engines don't like to run inverted. I know of one that blew up that way. If you know that the valve cover is filling with oil then I would think it can't return to the sump. That will cause issues. More oil in it will only cause more oil to pool where it shouldn't. Best to install it upright . I know that the Briggs engines will run inverted during a maneauver but the plane returns to upright allowing the oil to return to it's sump. Main reason I did the Honda GX-31 for my current project.